1. Field of the Invention
The present invention relates to an optical transmitter in which optical transmission paths are optically coupled to another optical transmission path.
2. Description of the Related Art
Jpn. Pat. Appln. KOKAI Publication No. 2005-283917 discloses one example of a conventional optical transmitter. This optical transmitter is described with reference to FIG. 8 to FIG. 9. In this optical transmitter, light source holders 55, 56, and 57 are provided for emission optical transmission paths 50, 51, and 52 on one end. The light source holders 55, 56, and 57 are optically coupled to light sources 58, 59, and 60, respectively. The emission optical transmission paths 50, 51, and 52 are enclosed together on the other end by a single ferrule 53. The end faces of the emission optical transmission paths 50, 51, and 52 are provided to meet given positions in the end face of the ferrule 53. A reception transmission optical path 54 is located near the end face of the ferrule 53. The end face of the reception transmission optical path 54 faces the end faces of the emission optical transmission paths 50, 51, and 52. The emission optical transmission paths 50, 51, and 52 and the reception transmission optical path 54 are constituted by so-called optical fibers. Each of the optical fiber has a core serving as an optical path, and a clad enclosing the core. Light rays emitted from the light sources 58, 59, and 60 are guided into the emission optical transmission paths 50, 51, and 52, propagate through the emission optical transmission paths 50, 51, and 52, exit from the end faces of the emission optical transmission paths 50, 51, and 52, and enter the core of the reception transmission optical path 54, respectively.
An enclosing core diameter occupying the cores of the emission optical transmission paths 50, 51, and 52 exposed in the end face of the ferrule 53 at the end of an emission optical transmission path is equal to or less than the core diameter of the reception transmission path. Moreover, a numerical aperture (NA) determined by the refractive indexes of the core and clad of the reception transmission path is equal to or more than the maximum numerical aperture (NA) of each of the transmission paths 50, 51, and 52.
The ends of the emission optical transmission paths 50, 51, and 52 are enclosed by the single ferrule 53, and the end faces of the reception transmission optical paths are provided to meet the given positions in the end face of the ferrule. Thus, advantageously, the ends of the emission transmission optical paths are easy to handle and can be small-sized.
However, for structural reasons, the cores of the transmission paths 50, 51, and 52 cannot be brought closer beyond a certain distance because of the interference of the outside diameters of the clads. That is, for example, as shown in FIG. 10, the cores of the emission optical transmission paths 50 and 51 are apart from one another as much as at least the thickness of the clads of the emission optical transmission paths 50 and 51 even in a configuration in which the emission optical transmission paths 50 and 51 are adjacent to one another. Therefore, the reception transmission optical path 54 having a suitable core diameter can only be used. As a result, the reception transmission optical path is prevented from being reduced in diameter.
In the example shown in FIG. 11, the emission optical transmission paths 51 are arranged around the emission optical transmission path 50, and the ends of the emission optical transmission paths 50 and 51 are held by the ferrule 53. The reception transmission optical path 54 comprises two cores 54a and 54b that are coaxially disposed, and a clad 54c enclosing these cores. The end face of the reception transmission optical path 54 is disposed to face the end face of the ferrule 53. In this case as well, the core diameter of the reception transmission optical path 54 is larger than an enclosing core diameter occupying the cores of the emission optical transmission paths 50 and 51.
Furthermore, in the example shown in FIG. 12, the reception transmission optical path 54 has a structure in which a single core 54d is enclosed by a clad 54e, and the peripheral edge of the end face of the ferrule 53 is slanted. In this configuration, a light ray exiting from the outside emission optical transmission path 51 exits inwardly at a slant. Thus, the reception transmission optical path 54 can be reduced in diameter as compared with the example in FIG. 11, but cannot be drastically reduced in diameter when coupling efficiency, for example, is considered.